1. Field of the Invention
The present invention relates to an air filter used for removing ionized gaseous pollutants, total organic compound (TOC), moisture, ozone, and other foreign matter with an abnormal odor from the air in clean rooms and the like, an air filter sheet for manufacturing the air filter, and a method for manufacturing the same. More particularly, the present invention relates to an air filter possessing aeration cavities and carrying a functional agent, a sheet for manufacturing the air filter, and a process for manufacturing the same.
2. Background Art
In frontier industries such as the semiconductor manufacturing industry and liquid crystal display manufacturing industry, controlling pollution of the air and product surfaces in clean rooms to prevent pollution is important to ensure a high yield, high quality, and reliability of the products. In the semiconductor manufacturing industry, in particular, as the degree of integration of the products increases, control of ionized gaseous pollutants, TOC, moisture, ozone, and other foreign matters with an abnormal odor has become indispensable in addition to the control of particulate matter using a HEPA filter, a ULPA filter, and the like.
The ionized gaseous pollutants include basic gases and acidic gases. Of these gases, ammonia, which is a basic gas, is known to adversely affect developability during the step of exposure to radiation and to cause wafer surfaces to become clouded in the manufacture of semiconductors. SOX, which is an acidic gas, produces lamination defects in substrates in a thermal oxidation membrane-forming step during manufacture of semiconductors, whereby the characteristics and reliability of the semiconductor devices are adversely affected.
Since ionized gaseous pollutants cause various problems in semiconductor manufacturing processes and the like, the concentration of ionized gaseous pollutants in a clean room used in semiconductor manufacturing and the like is required to be reduced to several micron grams per cubic meter (μg/m3).
For this reason, air filters made from a fibrous carrier with aeration cavities, which carries an ion-exchanger as a functional agent, have been conventionally used for removing ionized gaseous materials. A conventionally used air filter will be described referring to FIG. 5. FIG. 5 is a schematic diagram showing a conventional air filter with aeration cavities. The air filter 20 shown in FIG. 5 is fabricated by inserting an ion-exchange resin-carrying fiber material 25, which is obtained by causing an ion-exchanger to be carried on a fibrous carrier formed by alternately laminating a corrugated fiber 21 and a flat fiber 22, into a frame 26. Aeration cavities 23 are formed between the corrugated fiber 21 and the flat fiber 22. Gas to be processed 27 is caused to pass through the air filter 20 in the air flow direction 24, whereby ionized gaseous pollutants in the gas to be processed 27 is adsorbed on the filter and removed. In this instance, the direction in which the aeration cavities 23 are formed is parallel to the air flow direction 24.
Moisture must be removed, because the moisture participates in the formation of oxide films which cause wafer malfunctions. To remove moisture, an air filter carrying a dehumidification agent has been used. Ozone generated in an apparatus in a clean room operated at a high voltage must also be removed, because the ozone produces an oxide film on wafers. To remove ozone, an air filter carrying an ozone decomposition catalyst has been used.
In the above air filter, the functional agent such as an ion-exchanger is carried by an inorganic or organic binder. To cause the functional agent to be carried on a fibrous carrier, the carrier must be impregnated with a slurry prepared by mixing the functional agent with a binder, or such a slurry must be attached to the fibrous carrier. An excess amount of the binder may cover the surface of the functional agent carried on the fibrous carrier, impairing the adsorption performance of the air filter.
Since the above air filter is demanded not only to exhibit excellent initial removing performance, but also to maintain the excellent initial removing performance for a long period, in other words, to possess outstanding durability, the air filter must carry a large amount of functional agent thereon.
To this end, the surface area of the fibrous carrier material on which the functional agent is carried must be increased by increasing the number of aeration cavities per unit volume of the air filter. To increase the number of aeration cavities the cross-section area of openings of the aeration cavities must be decreased. However, if the cross-section area of one aeration cavities is too small, the slurry cannot enter the aeration cavities during the operation of carrying the functional agent on the fibrous material or the functional agent may clog the aeration cavities.
This problem can be solved if the functional agent can be carried on an air filter without using a binder.
As the method for causing the functional agent to be carried without using a binder, a method of using a polytetrafluoroethylene resin in the form of fibril (hereinafter referred to from time to time as “fibrillated polytetrafluoroethylene resin”) and causing the polytetrafluoroethylene resin to capture the substance to be carried has been used. For example, JP-A-2003-220333 discloses an ammonia gas capturing composite material comprising a solid capturing material capable of capturing the ammonia gas and a fibrillated polytetrafluoroethylene resin. The solid capturing material is held by the polytetrafluoroethylene resin so that ammonia gas is capable of reaching the solid capturing material, whereas a liquid is incapable of reaching the solid capturing material.
JP-A-2003-300066 discloses a composite powdery material for capturing ammonia gas comprising an ammonia gas capturing powder covered with a polytetrafluoroethylene resin having a number average molecular weight of 200,000 to 2,000,000.
In the manufacture of conventional air filters in which a functional agent is captured by a fibrillated polytetrafluoroethylene resin such as the composite capturing material described in JP-A-2003-220333, the fibrillated polytetrafluoroethylene resin is prepared by applying a shearing stress to a polytetrafluoroethylene resin. However, when applying a shearing stress to a polytetrafluoroethylene resin in this method, fibrils of the fibrillated polytetrafluoroethylene resin are vigorously entangled with each other to produce lumps if a lubricant such as a solvent naphtha is not added. Therefore, mixing a lubricant is essential. For this reason, conventional air filters contain a lubricant.
However, if an air filter contains a lubricant, the lubricant mixes in the air passing through the air filter (hereinafter referred to from time to time as “outgas”), thereby polluting the outgas. Since even a very small amount of lubricant in the air may affect the product quality when the air is used in clean rooms particularly for manufacturing semiconductors, liquid-crystals, and the like, air filters containing a lubricant cannot be used.
JP-A-2003-300066 states that a polytetrafluoroethylene resin having a molecular weight in the range of 200,000 to 2,000,000 can be fibrillated without adding a lubricant. However, a sheet obtained from such a polytetrafluoroethylene resin is too tender due to the small molecular weight to maintain a configuration (such as a corrugate) of the air filter when formed into that configuration. Therefore, the sheet possesses only very poor shape retainability.
An object of the present invention is therefore to provide an air filter sheet comprising fibrils of polytetrafluoroethylene resin with a functional agent carried thereon, possessing excellent formability, and being free from contamination of outgas with a lubricant, and a process for manufacturing the sheet. Another object of the present invention is to provide an air filter formed from the sheet comprising fibrils of polytetrafluoroethylene resin with a functional agent carried on the fibers, being free from contamination with a lubricant.
As a result of extensive studies to solve the above problems in the conventional technologies, the inventors of the present invention have discovered that if particles of a functional agent with a specific particle size are added to particles of a polytetrafluoroethylene resin, the polytetrafluoroethylene resin can be excellently fibrillated without using a lubricant, because the functional agent acts as a lubricant in the mixture.